why are the sound waves invisible
Sound waves are not always invisible. Sound is simply the ordered vibration of matter (as opposed to heat, which is the disordered vibration of matter). Any time you see a vibration, you are seeing sound. For instance, hit a bell and watch it shake. That shaking motion of the bell is the sound waves traveling through the bell. When a jet flies close by over your house, you hear a rumble and see the silverware dance around on the table. That dancing motion is the sound waves from the jet traveling through your table and silverware.